This invention relates to a circuit for isolating digital logic signals, such as signals in a process control system.
In process control systems, logic signals are transmitted between equipment and microcontrollers to provide status information to the microcontrollers, and to provide control information to the equipment. The microcontrollers and the equipment should be electronically isolated so that transient signals do not inadvertently trigger erroneous status or control information. A well known method for achieving such isolation is to use optical isolators that convert digital logic signals to light pulses generated by light emitting diodes (LEDs), and then to convert the received light pulses back into digital logic signals. Optical isolators have a number of drawbacks, however: they require significant space on a card or circuit board, they draw a large current, they do not operate well at high frequency, and they are very inefficient.
An isolation amplifier that avoids the use of such optical couplers is described in Somerville, U.S. Pat. No. 4,748,419. In that patent, an input data signal is differentiated to create a pair of differential signals that are each transmitted across respective high voltage capacitors to create differentiated spike signals for the differential input pair. Circuitry on the other side of the capacitive barrier has a differential amplifier, a pair of converters for comparing the amplified signal against high and low thresholds, and a set/reset flip-flop to restore the spikes created by the capacitors back into a logic signal. In such a capacitively-coupled device, however, during a common mode transient event, the capacitors couple high, common mode energy into the receiving circuit. As the rate of voltage change increases in that common mode event, the current injected into the receiver increases. This current can potentially damage the receiving circuit and can trigger a faulty detection. Such capacitively coupled circuitry thus couples signals that should be rejected. The patent also mentions, without elaboration, that a transformer with a short R/L time constant can provide an isolation barrier, but such a differential approach is nonetheless undesirable because any mismatch in the non-magnetic (capacitive) coupling of the windings would cause a common-mode signal to appear as a difference signal.